1. Field of the Invention
The present invention relates to methods and apparatuses for producing a powder compact using powders for the production of permanent magnets, soft magnetic materials, electric and electronic materials, tool materials, and various parts used in a variety of industrial fields. The powder packed in a rubber mold is pressed by using the pressure of the punches of a die press machine and deformation of rubber so as to produce a powder compact. This type of pressing is hereinafter referred to as "rubber mold die pressing."
2. Discussion of the Related Art
One of the known pressing technologies is the rubber mold die pressing method disclosed in Japanese Unexamined Patent Publication No. 55-26601. In this method, a rubber mold is loaded in a die secured to the table of a die press machine, and the cavity provided in the rubber mold is filled with a rare-earth cobalt alloy powder. The upper punch is subsequently lowered to carry out the die pressing. By using a rubber mold as a pressure medium, this method made it possible to produce rare-earth cobalt magnets with improved magnetic properties as high as those by transverse die pressing.
Referring to the prior rubber mold die pressing methods mentioned above, as well as to FIG. 8 illustrating the apparatus to implement those methods, the problems that the present invention intends to solve are described below.
Referring to FIG. 8, an indexed table 1 turns intermittently, driven by an appropriate means provided in a base table 2. A described number of dies M, which number is determined according to the number of steps for the production of compacts, are placed on the indexed table 1. In FIG. 8, only one die M placed at the die pressing stage is shown, while the dies assigned for other stages are omitted.
Die M comprises a cylindrical die d (hereinafter simply referred to as the "die") fixed to the indexed table 1 and a lower punch p1 with a height less than die d is inserted into the die. A rubber mold m is loaded, in recess 3 (hereinafter referred to as the "die cavity") formed by die d and lower punch p1 inserted in the die. Rubber mold m is provided with a recess (hereinafter referred to as the "rubber cavity") which is designed in accordance with the desired configuration of the compact to be formed.
Rubber cavity m' is filled with a powder in the process on the indexed table 1. Then die M, whose cavity 3 is loaded with the rubber mold m filled with powder, is transferred to the die pressing stage by the intermittent turning of the indexed table 1 as shown in FIG. 8.
An upper punch p2 is inserted into die d attached to the end of a cylinder rod 5 of a fluid cylinder 4. A magnetic field generator coil 6 is connected with the cylinder rod 5 or the like. When a magnetic powder is oriented to produce permanent magnets, the cylinder rod 5 or the like is provided with the magnetic field generator coil 6. A horizontal frame is used to locate the fluid cylinder 4. The horizontal frame 7 is mounted on vertical frames 8 and 8' placed on the floor f.
In the die press machine constructed as above, after the positioning of the upper punch p2 and the die cavity 3, the upper punch p2 is inserted into the die d, and then the fluid cylinder 4 works to push down the upper punch p2 so that rubber mold m filled with powder is pressed in the space formed by die d, lower punch p1 both of which are secured to the base of the die press machine and the upper punch p2. Due to the downward movement of the upper punch p2 and inward shrinking of rubber cavity m', the thickness of the rubber mold m is reduced, thereby producing a powder compact from the powder packed in the rubber mold.
The conventional rubber mold die pressing method must include an accurate positioning for die M to be transferred to the die pressing stage by the intermittent turning of indexed table 1, because the upper punch p2 needs to be inserted into die cavity 3. Accordingly, the intermittent turning system of the indexed table 1 must be provided with the function of accurate positioning, which makes the apparatus very complex and expensive. Also, this type of apparatus requires frequent checks and maintenance.
If a clearance exists between the outer circumference of the upper punch p2 and the inner circumference of die d, the rubber mold m gets caught in this clearance upon pressing. This causes many problems such as damage to the rubber mold, ascent of the rubber mold when pulling out the upper punch from the die, and cracks and fractures in the compact due to the unnecessary force placed on the rubber mold.
For the reasons above, the upper punch p2 and the die d must be made as precisely as possible. However, the more precisely the upper punch and the die are fabricated, the more accurate positioning must be carried out by the intermittent turning system table 1.
The need for more accurate positioning complicates the turning system and the construction of the die press machine, which causes more disorders, and therefore requires more frequent maintenance.
In the apparatus above, when the upper punch p2 is lowered, the rubber mold m is pressed in the space formed by the die d, the lower punch p1 and the upper punch p2. Following pressing, the thickness of the rubber mold m is reduced and the rubber cavity m' shrinks inwardly, while due to the compression of rubber mold by the lowering of upper punch p2, rubber mold m works to press the inner wall of the die d, causing the generation of a very large frictional force between the rubber mold m and the die d.
The very large frictional force generated between the die d and the rubber mold m affects the compressing power applied to the powder filling the rubber cavity. This is especially true when the rubber cavity m' is deep. This is because while the upper part of the rubber cavity receives sufficient compressing power, the part deep in the rubber cavity cannot get enough power. As a result, the powder compact is not uniform in density, and it is likely to break apart when insufficiently pressed.